Properly handle edge cases in AN10922 key diversification #118
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smortex
reviewed
Oct 29, 2019
smortex
reviewed
Oct 29, 2019
This commit fixes issue nfc-tools#91. [AN10922][] specifies the key diversification algorithms used by the MIFARE SAM AV3. Support for these algorithms was added to `libfreefare` via pull-request nfc-tools#79. However, while every attempt was made to write a faithful implementation, the implemented code did not properly handle cases where the diversification data was less than or equal to the block size of the cipher: 16 bytes for AES, and 8 bytes for DES. This bug was identified in issue nfc-tools#91. This commit addresses this problem while providing a way to revert to the previous behavior in cases where it is necessary to maintain previous deployments. This was accomplished by introducing a new `flags` parameter to the `mifare_key_deriver_new_an10922` method. Normally, `flags` should simply be set to `AN10922_FLAG_DEFAULT`. However, if the previous behavior is required, it should be set to `AN10922_FLAG_EMULATE_ISSUE_91`. [AN10922][] does not include any test vectors that might have helped to identify this problem earlier. However, [AN10957][] (pages 13-14) was found to have a suitable example usage of [AN10922][] with an appropriately short value for *M* that we are using as a test vector to verify correct behavior. Note that the issue being addressed here is not a security issue: using the `AN10922_FLAG_EMULATE_ISSUE_91` should not be any less secure than using `AN10922_FLAG_DEFAULT`. [AN10922]: https://www.nxp.com/docs/en/application-note/AN10922.pdf [AN10957]: https://www.nxp.com/docs/en/application-note/AN10957.pdf
smortex
approved these changes
Oct 29, 2019
This was referenced Oct 29, 2019
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This commit fixes issue #91.
AN10922 specifies the key diversification algorithms used by the MIFARE SAM AV3. Support for these algorithms was added to
libfreefarevia pull-request #79.However, while every attempt was made to write a faithful implementation, the implemented code did not properly handle cases where the diversification data was less than or equal to the block size of the cipher: 16 bytes for AES, and 8 bytes for DES. This bug was identified in issue #91.
This commit addresses this problem while providing a way to revert to the previous behavior in cases where it is necessary to maintain previous deployments. This was accomplished by introducing a new
flagsparameter to themifare_key_deriver_new_an10922method.Normally,
flagsshould simply be set toAN10922_FLAG_DEFAULT. However, if the previous behavior is required, it should be set toAN10922_FLAG_EMULATE_ISSUE_91.AN10922 does not include any test vectors that might have helped to identify this problem earlier. However, AN10957 (pages 13-14) was found to have a suitable example usage of AN10922 with an appropriately short value for M that we are using as a test vector to verify correct behavior.
Note that the issue being addressed here is not a security issue: using the
AN10922_FLAG_EMULATE_ISSUE_91should not be any less secure than usingAN10922_FLAG_DEFAULT.