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mbedtls_ecp_write_key: document and test smaller output buffer

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Document and test the current behavior, even if it is weird:

* For Weierstrass keys, the error is MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL,
  not MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL.
* For Weierstrass keys, a smaller output buffer is ok if the output fits.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine
2024-02-19 13:49:45 +01:00
parent a395bdd066
commit 1c7ff7ea53
3 changed files with 118 additions and 12 deletions
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15
include/mbedtls/ecp.h
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@ -1331,8 +1331,8 @@ int mbedtls_ecp_read_key(mbedtls_ecp_group_id grp_id, mbedtls_ecp_keypair *key,
* \brief This function exports an elliptic curve private key.
*
* \note Note that although this function accepts an output
* buffer that is larger than the key, most key import
* interfaces require the output to be trimmed to the
* buffer that is smaller or larger than the key, most key
* import interfaces require the output to have exactly
* key's nominal length. It is generally simplest to
* pass the key's nominal length as \c buflen, after
* checking that the output buffer is large enough.
@ -1349,13 +1349,18 @@ int mbedtls_ecp_read_key(mbedtls_ecp_group_id grp_id, mbedtls_ecp_keypair *key,
* representation (which is little-endian), padded with
* null bytes at the end to reach \p buflen bytes.
* \param buflen The total length of the buffer in bytes.
* The length of the output is always
* The length of the output is
* (`grp->nbits` + 7) / 8 bytes
* where `grp->nbits` is the private key size in bits.
* For Weierstrass keys, if the output buffer is smaller,
* leading zeros are trimmed to fit if possible. For
* Montgomery keys, the output buffer must always be large
* enough for the nominal length.
*
* \return \c 0 on success.
* \return #MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL if the \p key
representation is larger than the available space in \p buf.
* \return #MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL or
* #MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL if the \p key
* representation is larger than the available space in \p buf.
* \return Another negative error code on different kinds of failure.
*/
int mbedtls_ecp_write_key(mbedtls_ecp_keypair *key,