pqc_kyber

The crate has had no releases since 0.7.1 (2023-08-23), and the upstream repository shows no maintainer activity. Open pull requests, including a fix for a chosen-ciphertext key-recovery flaw in the AVX2 backend (Argyle-Software/kyber#121), have gone unanswered.

Recommended alternatives:

• aws-lc-rs
• graviola

Summary

When pqc_kyber is built with the opt-in avx2 feature on x86_64, its constant-time conditional-move routine cmov is a no-op: it never performs the copy. That cmov is exactly the step that applies the Fujisaki-Okamoto (FO) implicit rejection during decapsulation. With it disabled, decapsulating an invalid ciphertext no longer returns a key-independent pseudorandom value; it returns a value that depends on the decrypted plaintext of the attacker-chosen ciphertext. This restores the chosen-ciphertext decryption oracle that the FO transform exists to remove, and yields full recovery of the static secret key.

The attack requires no timing measurement, no side channel, and no faults; only the ability to submit ciphertexts to a decapsulation operation under a reused key pair and observe the resulting shared secret. See the linked pull request for the root cause, the reachable call path, and reproduction details.

Affected configuration

Reachable only in builds with features = ["avx2"] on an x86_64 target. The default (reference) backend and all non-x86_64 targets are unaffected.

Impact

Complete IND-CCA break. The plaintext-checking oracle drives a standard chosen-ciphertext key-recovery attack and extracts the entire static secret key. Any protocol reusing a Kyber key pair across decapsulations (static or long-term KEM keys, KEMTLS, HPKE recipients, pinned keys) is exposed; ephemeral-only key shares are not. The defect was verified end to end on ML-KEM-768: the full secret key was recovered in 4,272 decapsulation queries.

Maintenance status

The crate is unmaintained (last release 0.7.1, August 2023); no patched version exists or is expected.

Mitigation

• If remaining on this crate, do not enable the avx2 feature; the default reference backend performs implicit rejection correctly.
• Do not reuse a Kyber key pair across decapsulations.

Credit

Reported by 007bsd.

Various Kyber software libraries in various environments leak secret information into timing, specifically because

• these libraries include a line of code that divides a secret numerator by a public denominator,
• the number of CPU cycles for division in various environments varies depending on the inputs to the division, and
• this variation appears within the range of numerators used in these libraries.

The KyberSlash pages track which Kyber libraries have this issue, and include a FAQ about the issue.

Author

The KyberSlash pages were written by Daniel J. Bernstein. The FAQ originally said "I", but some people seemed to have trouble finding this authorship statement, so the FAQ now says "Bernstein" instead.

URL

The permanent link for the KyberSlash pages is https://kyberslash.cr.yp.to.

Mitigation status in pqc_kyber crate

The issues has not been resolved in the pqc_kyber crate. A third-party fork that mitigates this attack vector has been published as safe_pqc_kyber.

The crate has had no releases since 0.7.1 (2023-08-23), and the upstream repository shows no maintainer activity. Open pull requests, including a fix for a chosen-ciphertext key-recovery flaw in the AVX2 backend (Argyle-Software/kyber#121), have gone unanswered.

Recommended alternatives:

• aws-lc-rs
• graviola

Summary

When pqc_kyber is built with the opt-in avx2 feature on x86_64, its constant-time conditional-move routine cmov is a no-op: it never performs the copy. That cmov is exactly the step that applies the Fujisaki-Okamoto (FO) implicit rejection during decapsulation. With it disabled, decapsulating an invalid ciphertext no longer returns a key-independent pseudorandom value; it returns a value that depends on the decrypted plaintext of the attacker-chosen ciphertext. This restores the chosen-ciphertext decryption oracle that the FO transform exists to remove, and yields full recovery of the static secret key.

The attack requires no timing measurement, no side channel, and no faults; only the ability to submit ciphertexts to a decapsulation operation under a reused key pair and observe the resulting shared secret. See the linked pull request for the root cause, the reachable call path, and reproduction details.

Affected configuration

Reachable only in builds with features = ["avx2"] on an x86_64 target. The default (reference) backend and all non-x86_64 targets are unaffected.

Impact

Complete IND-CCA break. The plaintext-checking oracle drives a standard chosen-ciphertext key-recovery attack and extracts the entire static secret key. Any protocol reusing a Kyber key pair across decapsulations (static or long-term KEM keys, KEMTLS, HPKE recipients, pinned keys) is exposed; ephemeral-only key shares are not. The defect was verified end to end on ML-KEM-768: the full secret key was recovered in 4,272 decapsulation queries.

Maintenance status

The crate is unmaintained (last release 0.7.1, August 2023); no patched version exists or is expected.

Mitigation

• If remaining on this crate, do not enable the avx2 feature; the default reference backend performs implicit rejection correctly.
• Do not reuse a Kyber key pair across decapsulations.

Credit

Reported by 007bsd.

Various Kyber software libraries in various environments leak secret information into timing, specifically because

• these libraries include a line of code that divides a secret numerator by a public denominator,
• the number of CPU cycles for division in various environments varies depending on the inputs to the division, and
• this variation appears within the range of numerators used in these libraries.

The KyberSlash pages track which Kyber libraries have this issue, and include a FAQ about the issue.

Author

The KyberSlash pages were written by Daniel J. Bernstein. The FAQ originally said "I", but some people seemed to have trouble finding this authorship statement, so the FAQ now says "Bernstein" instead.

URL

The permanent link for the KyberSlash pages is https://kyberslash.cr.yp.to.

Mitigation status in pqc_kyber crate

The issues has not been resolved in the pqc_kyber crate. A third-party fork that mitigates this attack vector has been published as safe_pqc_kyber.

The crate has had no releases since 0.7.1 (2023-08-23), and the upstream repository shows no maintainer activity. Open pull requests, including a fix for a chosen-ciphertext key-recovery flaw in the AVX2 backend (Argyle-Software/kyber#121), have gone unanswered.

Recommended alternatives:

• aws-lc-rs
• graviola

Summary

When pqc_kyber is built with the opt-in avx2 feature on x86_64, its constant-time conditional-move routine cmov is a no-op: it never performs the copy. That cmov is exactly the step that applies the Fujisaki-Okamoto (FO) implicit rejection during decapsulation. With it disabled, decapsulating an invalid ciphertext no longer returns a key-independent pseudorandom value; it returns a value that depends on the decrypted plaintext of the attacker-chosen ciphertext. This restores the chosen-ciphertext decryption oracle that the FO transform exists to remove, and yields full recovery of the static secret key.

The attack requires no timing measurement, no side channel, and no faults; only the ability to submit ciphertexts to a decapsulation operation under a reused key pair and observe the resulting shared secret. See the linked pull request for the root cause, the reachable call path, and reproduction details.

Affected configuration

Reachable only in builds with features = ["avx2"] on an x86_64 target. The default (reference) backend and all non-x86_64 targets are unaffected.

Impact

Complete IND-CCA break. The plaintext-checking oracle drives a standard chosen-ciphertext key-recovery attack and extracts the entire static secret key. Any protocol reusing a Kyber key pair across decapsulations (static or long-term KEM keys, KEMTLS, HPKE recipients, pinned keys) is exposed; ephemeral-only key shares are not. The defect was verified end to end on ML-KEM-768: the full secret key was recovered in 4,272 decapsulation queries.

Maintenance status

The crate is unmaintained (last release 0.7.1, August 2023); no patched version exists or is expected.

Mitigation

• If remaining on this crate, do not enable the avx2 feature; the default reference backend performs implicit rejection correctly.
• Do not reuse a Kyber key pair across decapsulations.

Credit

Reported by 007bsd.

Various Kyber software libraries in various environments leak secret information into timing, specifically because

• these libraries include a line of code that divides a secret numerator by a public denominator,
• the number of CPU cycles for division in various environments varies depending on the inputs to the division, and
• this variation appears within the range of numerators used in these libraries.

The KyberSlash pages track which Kyber libraries have this issue, and include a FAQ about the issue.

Author

The KyberSlash pages were written by Daniel J. Bernstein. The FAQ originally said "I", but some people seemed to have trouble finding this authorship statement, so the FAQ now says "Bernstein" instead.

URL

The permanent link for the KyberSlash pages is https://kyberslash.cr.yp.to.

Mitigation status in pqc_kyber crate

The issues has not been resolved in the pqc_kyber crate. A third-party fork that mitigates this attack vector has been published as safe_pqc_kyber.

The crate has had no releases since 0.7.1 (2023-08-23), and the upstream repository shows no maintainer activity. Open pull requests, including a fix for a chosen-ciphertext key-recovery flaw in the AVX2 backend (Argyle-Software/kyber#121), have gone unanswered.

Recommended alternatives:

• aws-lc-rs
• graviola

Summary

When pqc_kyber is built with the opt-in avx2 feature on x86_64, its constant-time conditional-move routine cmov is a no-op: it never performs the copy. That cmov is exactly the step that applies the Fujisaki-Okamoto (FO) implicit rejection during decapsulation. With it disabled, decapsulating an invalid ciphertext no longer returns a key-independent pseudorandom value; it returns a value that depends on the decrypted plaintext of the attacker-chosen ciphertext. This restores the chosen-ciphertext decryption oracle that the FO transform exists to remove, and yields full recovery of the static secret key.

The attack requires no timing measurement, no side channel, and no faults; only the ability to submit ciphertexts to a decapsulation operation under a reused key pair and observe the resulting shared secret. See the linked pull request for the root cause, the reachable call path, and reproduction details.

Affected configuration

Reachable only in builds with features = ["avx2"] on an x86_64 target. The default (reference) backend and all non-x86_64 targets are unaffected.

Impact

Complete IND-CCA break. The plaintext-checking oracle drives a standard chosen-ciphertext key-recovery attack and extracts the entire static secret key. Any protocol reusing a Kyber key pair across decapsulations (static or long-term KEM keys, KEMTLS, HPKE recipients, pinned keys) is exposed; ephemeral-only key shares are not. The defect was verified end to end on ML-KEM-768: the full secret key was recovered in 4,272 decapsulation queries.

Maintenance status

The crate is unmaintained (last release 0.7.1, August 2023); no patched version exists or is expected.

Mitigation

• If remaining on this crate, do not enable the avx2 feature; the default reference backend performs implicit rejection correctly.
• Do not reuse a Kyber key pair across decapsulations.

Credit

Reported by 007bsd.

Various Kyber software libraries in various environments leak secret information into timing, specifically because

• these libraries include a line of code that divides a secret numerator by a public denominator,
• the number of CPU cycles for division in various environments varies depending on the inputs to the division, and
• this variation appears within the range of numerators used in these libraries.

The KyberSlash pages track which Kyber libraries have this issue, and include a FAQ about the issue.

Author

The KyberSlash pages were written by Daniel J. Bernstein. The FAQ originally said "I", but some people seemed to have trouble finding this authorship statement, so the FAQ now says "Bernstein" instead.

URL

The permanent link for the KyberSlash pages is https://kyberslash.cr.yp.to.

Mitigation status in pqc_kyber crate

The issues has not been resolved in the pqc_kyber crate. A third-party fork that mitigates this attack vector has been published as safe_pqc_kyber.

The crate has had no releases since 0.7.1 (2023-08-23), and the upstream repository shows no maintainer activity. Open pull requests, including a fix for a chosen-ciphertext key-recovery flaw in the AVX2 backend (Argyle-Software/kyber#121), have gone unanswered.

Recommended alternatives:

• aws-lc-rs
• graviola

Summary

When pqc_kyber is built with the opt-in avx2 feature on x86_64, its constant-time conditional-move routine cmov is a no-op: it never performs the copy. That cmov is exactly the step that applies the Fujisaki-Okamoto (FO) implicit rejection during decapsulation. With it disabled, decapsulating an invalid ciphertext no longer returns a key-independent pseudorandom value; it returns a value that depends on the decrypted plaintext of the attacker-chosen ciphertext. This restores the chosen-ciphertext decryption oracle that the FO transform exists to remove, and yields full recovery of the static secret key.

The attack requires no timing measurement, no side channel, and no faults; only the ability to submit ciphertexts to a decapsulation operation under a reused key pair and observe the resulting shared secret. See the linked pull request for the root cause, the reachable call path, and reproduction details.

Affected configuration

Reachable only in builds with features = ["avx2"] on an x86_64 target. The default (reference) backend and all non-x86_64 targets are unaffected.

Impact

Complete IND-CCA break. The plaintext-checking oracle drives a standard chosen-ciphertext key-recovery attack and extracts the entire static secret key. Any protocol reusing a Kyber key pair across decapsulations (static or long-term KEM keys, KEMTLS, HPKE recipients, pinned keys) is exposed; ephemeral-only key shares are not. The defect was verified end to end on ML-KEM-768: the full secret key was recovered in 4,272 decapsulation queries.

Maintenance status

The crate is unmaintained (last release 0.7.1, August 2023); no patched version exists or is expected.

Mitigation

• If remaining on this crate, do not enable the avx2 feature; the default reference backend performs implicit rejection correctly.
• Do not reuse a Kyber key pair across decapsulations.

Credit

Reported by 007bsd.

Various Kyber software libraries in various environments leak secret information into timing, specifically because

• these libraries include a line of code that divides a secret numerator by a public denominator,
• the number of CPU cycles for division in various environments varies depending on the inputs to the division, and
• this variation appears within the range of numerators used in these libraries.

The KyberSlash pages track which Kyber libraries have this issue, and include a FAQ about the issue.

Author

The KyberSlash pages were written by Daniel J. Bernstein. The FAQ originally said "I", but some people seemed to have trouble finding this authorship statement, so the FAQ now says "Bernstein" instead.

URL

The permanent link for the KyberSlash pages is https://kyberslash.cr.yp.to.

Mitigation status in pqc_kyber crate

The issues has not been resolved in the pqc_kyber crate. A third-party fork that mitigates this attack vector has been published as safe_pqc_kyber.

The crate has had no releases since 0.7.1 (2023-08-23), and the upstream repository shows no maintainer activity. Open pull requests, including a fix for a chosen-ciphertext key-recovery flaw in the AVX2 backend (Argyle-Software/kyber#121), have gone unanswered.

Recommended alternatives:

• aws-lc-rs
• graviola

Summary

When pqc_kyber is built with the opt-in avx2 feature on x86_64, its constant-time conditional-move routine cmov is a no-op: it never performs the copy. That cmov is exactly the step that applies the Fujisaki-Okamoto (FO) implicit rejection during decapsulation. With it disabled, decapsulating an invalid ciphertext no longer returns a key-independent pseudorandom value; it returns a value that depends on the decrypted plaintext of the attacker-chosen ciphertext. This restores the chosen-ciphertext decryption oracle that the FO transform exists to remove, and yields full recovery of the static secret key.

The attack requires no timing measurement, no side channel, and no faults; only the ability to submit ciphertexts to a decapsulation operation under a reused key pair and observe the resulting shared secret. See the linked pull request for the root cause, the reachable call path, and reproduction details.

Affected configuration

Reachable only in builds with features = ["avx2"] on an x86_64 target. The default (reference) backend and all non-x86_64 targets are unaffected.

Impact

Complete IND-CCA break. The plaintext-checking oracle drives a standard chosen-ciphertext key-recovery attack and extracts the entire static secret key. Any protocol reusing a Kyber key pair across decapsulations (static or long-term KEM keys, KEMTLS, HPKE recipients, pinned keys) is exposed; ephemeral-only key shares are not. The defect was verified end to end on ML-KEM-768: the full secret key was recovered in 4,272 decapsulation queries.

Maintenance status

The crate is unmaintained (last release 0.7.1, August 2023); no patched version exists or is expected.

Mitigation

• If remaining on this crate, do not enable the avx2 feature; the default reference backend performs implicit rejection correctly.
• Do not reuse a Kyber key pair across decapsulations.

Credit

Reported by 007bsd.

Various Kyber software libraries in various environments leak secret information into timing, specifically because

• these libraries include a line of code that divides a secret numerator by a public denominator,
• the number of CPU cycles for division in various environments varies depending on the inputs to the division, and
• this variation appears within the range of numerators used in these libraries.

The KyberSlash pages track which Kyber libraries have this issue, and include a FAQ about the issue.

Author

The KyberSlash pages were written by Daniel J. Bernstein. The FAQ originally said "I", but some people seemed to have trouble finding this authorship statement, so the FAQ now says "Bernstein" instead.

URL

The permanent link for the KyberSlash pages is https://kyberslash.cr.yp.to.

Mitigation status in pqc_kyber crate

The issues has not been resolved in the pqc_kyber crate. A third-party fork that mitigates this attack vector has been published as safe_pqc_kyber.

The crate has had no releases since 0.7.1 (2023-08-23), and the upstream repository shows no maintainer activity. Open pull requests, including a fix for a chosen-ciphertext key-recovery flaw in the AVX2 backend (Argyle-Software/kyber#121), have gone unanswered.

Recommended alternatives:

• aws-lc-rs
• graviola

Summary

When pqc_kyber is built with the opt-in avx2 feature on x86_64, its constant-time conditional-move routine cmov is a no-op: it never performs the copy. That cmov is exactly the step that applies the Fujisaki-Okamoto (FO) implicit rejection during decapsulation. With it disabled, decapsulating an invalid ciphertext no longer returns a key-independent pseudorandom value; it returns a value that depends on the decrypted plaintext of the attacker-chosen ciphertext. This restores the chosen-ciphertext decryption oracle that the FO transform exists to remove, and yields full recovery of the static secret key.

The attack requires no timing measurement, no side channel, and no faults; only the ability to submit ciphertexts to a decapsulation operation under a reused key pair and observe the resulting shared secret. See the linked pull request for the root cause, the reachable call path, and reproduction details.

Affected configuration

Reachable only in builds with features = ["avx2"] on an x86_64 target. The default (reference) backend and all non-x86_64 targets are unaffected.

Impact

Complete IND-CCA break. The plaintext-checking oracle drives a standard chosen-ciphertext key-recovery attack and extracts the entire static secret key. Any protocol reusing a Kyber key pair across decapsulations (static or long-term KEM keys, KEMTLS, HPKE recipients, pinned keys) is exposed; ephemeral-only key shares are not. The defect was verified end to end on ML-KEM-768: the full secret key was recovered in 4,272 decapsulation queries.

Maintenance status

The crate is unmaintained (last release 0.7.1, August 2023); no patched version exists or is expected.

Mitigation

• If remaining on this crate, do not enable the avx2 feature; the default reference backend performs implicit rejection correctly.
• Do not reuse a Kyber key pair across decapsulations.

Credit

Reported by 007bsd.

Various Kyber software libraries in various environments leak secret information into timing, specifically because

• these libraries include a line of code that divides a secret numerator by a public denominator,
• the number of CPU cycles for division in various environments varies depending on the inputs to the division, and
• this variation appears within the range of numerators used in these libraries.

The KyberSlash pages track which Kyber libraries have this issue, and include a FAQ about the issue.

Author

The KyberSlash pages were written by Daniel J. Bernstein. The FAQ originally said "I", but some people seemed to have trouble finding this authorship statement, so the FAQ now says "Bernstein" instead.

URL

The permanent link for the KyberSlash pages is https://kyberslash.cr.yp.to.

Mitigation status in pqc_kyber crate

The issues has not been resolved in the pqc_kyber crate. A third-party fork that mitigates this attack vector has been published as safe_pqc_kyber.

The crate has had no releases since 0.7.1 (2023-08-23), and the upstream repository shows no maintainer activity. Open pull requests, including a fix for a chosen-ciphertext key-recovery flaw in the AVX2 backend (Argyle-Software/kyber#121), have gone unanswered.

Recommended alternatives:

• aws-lc-rs
• graviola

Summary

When pqc_kyber is built with the opt-in avx2 feature on x86_64, its constant-time conditional-move routine cmov is a no-op: it never performs the copy. That cmov is exactly the step that applies the Fujisaki-Okamoto (FO) implicit rejection during decapsulation. With it disabled, decapsulating an invalid ciphertext no longer returns a key-independent pseudorandom value; it returns a value that depends on the decrypted plaintext of the attacker-chosen ciphertext. This restores the chosen-ciphertext decryption oracle that the FO transform exists to remove, and yields full recovery of the static secret key.

The attack requires no timing measurement, no side channel, and no faults; only the ability to submit ciphertexts to a decapsulation operation under a reused key pair and observe the resulting shared secret. See the linked pull request for the root cause, the reachable call path, and reproduction details.

Affected configuration

Reachable only in builds with features = ["avx2"] on an x86_64 target. The default (reference) backend and all non-x86_64 targets are unaffected.

Impact

Complete IND-CCA break. The plaintext-checking oracle drives a standard chosen-ciphertext key-recovery attack and extracts the entire static secret key. Any protocol reusing a Kyber key pair across decapsulations (static or long-term KEM keys, KEMTLS, HPKE recipients, pinned keys) is exposed; ephemeral-only key shares are not. The defect was verified end to end on ML-KEM-768: the full secret key was recovered in 4,272 decapsulation queries.

Maintenance status

The crate is unmaintained (last release 0.7.1, August 2023); no patched version exists or is expected.

Mitigation

• If remaining on this crate, do not enable the avx2 feature; the default reference backend performs implicit rejection correctly.
• Do not reuse a Kyber key pair across decapsulations.

Credit

Reported by 007bsd.

Various Kyber software libraries in various environments leak secret information into timing, specifically because

• these libraries include a line of code that divides a secret numerator by a public denominator,
• the number of CPU cycles for division in various environments varies depending on the inputs to the division, and
• this variation appears within the range of numerators used in these libraries.

The KyberSlash pages track which Kyber libraries have this issue, and include a FAQ about the issue.

Author

The KyberSlash pages were written by Daniel J. Bernstein. The FAQ originally said "I", but some people seemed to have trouble finding this authorship statement, so the FAQ now says "Bernstein" instead.

URL

The permanent link for the KyberSlash pages is https://kyberslash.cr.yp.to.

Mitigation status in pqc_kyber crate

The issues has not been resolved in the pqc_kyber crate. A third-party fork that mitigates this attack vector has been published as safe_pqc_kyber.