PKCS7_verify, PKCS7_get0_signers - verify a PKCS#7 signedData structure
#include <openssl/pkcs7.h> int PKCS7_verify(PKCS7 *p7, STACK_OF(X509) *certs, X509_STORE *store, BIO *indata, BIO *out, int flags); STACK_OF(X509) *PKCS7_get0_signers(PKCS7 *p7, STACK_OF(X509) *certs, int flags);
PKCS7_verify() is very similar to CMS_verify(3). It verifies a PKCS#7 signedData structure given in p7. The optional certs parameter refers to a set of certificates in which to search for signer's certificates. p7 may contain extra untrusted CA certificates that may be used for chain building as well as CRLs that may be used for certificate validation. store may be NULL or point to the trusted certificate store to use for chain verification. indata refers to the signed data if the content is detached from p7. Otherwise indata should be NULL, and then the signed data must be in p7. The content is written to the BIO out unless it is NULL. flags is an optional set of flags, which can be used to modify the operation.
PKCS7_get0_signers() retrieves the signer's certificates from p7, it does not check their validity or whether any signatures are valid. The certs and flags parameters have the same meanings as in PKCS7_verify().
Normally the verify process proceeds as follows.
Initially some sanity checks are performed on p7. The type of p7 must be SignedData. There must be at least one signature on the data and if the content is detached indata cannot be NULL. If the content is not detached and indata is not NULL then the structure has both embedded and external content. To treat this as an error, use the flag PKCS7_NO_DUAL_CONTENT. The default behavior allows this, for compatibility with older versions of OpenSSL.
An attempt is made to locate all the signer's certificates, first looking in the certs parameter (if it is not NULL). Then they are looked up in any certificates contained in the p7 structure unless PKCS7_NOINTERN is set. If any signer's certificates cannot be located the operation fails.
Each signer's certificate is chain verified using the smimesign purpose and using the trusted certificate store store if supplied. Any internal certificates in the message, which may have been added using PKCS7_add_certificate(3), are used as untrusted CAs unless PKCS7_NOCHAIN is set. If CRL checking is enabled in store and PKCS7_NOCRL is not set, any internal CRLs, which may have been added using PKCS7_add_crl(3), are used in addition to attempting to look them up in store. If store is not NULL and any chain verify fails an error code is returned.
Finally the signed content is read (and written to out unless it is NULL) and the signature is checked.
If all signatures verify correctly then the function is successful.
Any of the following flags (ored together) can be passed in the flags parameter to change the default verify behaviour. Only the flag PKCS7_NOINTERN is meaningful to PKCS7_get0_signers().
If PKCS7_NOINTERN is set the certificates in the message itself are not searched when locating the signer's certificates. This means that all the signer's certificates must be in the certs parameter.
If PKCS7_NOCRL is set and CRL checking is enabled in store then any CRLs in the message itself are ignored.
If the PKCS7_TEXT flag is set MIME headers for type "text/plain" are deleted from the content. If the content is not of type "text/plain" then an error is returned.
If PKCS7_NOVERIFY is set the signer's certificates are not chain verified.
If PKCS7_NOCHAIN is set then the certificates contained in the message are not used as untrusted CAs. This means that the whole verify chain (apart from the signer's certificates) must be contained in the trusted store.
If PKCS7_NOSIGS is set then the signatures on the data are not checked.
One application of PKCS7_NOINTERN is to only accept messages signed by a small number of certificates. The acceptable certificates would be passed in the certs parameter. In this case if the signer's certificate is not one of the certificates supplied in certs then the verify will fail because the signer cannot be found.
Care should be taken when modifying the default verify behaviour, for example setting "PKCS7_NOVERIFY|PKCS7_NOSIGS" will totally disable all verification and any signed message will be considered valid. This combination is however useful if one merely wishes to write the content to out and its validity is not considered important.
Chain verification should arguably be performed using the signing time rather than the current time. However, since the signing time is supplied by the signer it cannot be trusted without additional evidence (such as a trusted timestamp).
PKCS7_verify() returns 1 for a successful verification and 0 if an error occurs.
PKCS7_get0_signers() returns all signers or NULL if an error occurred.
The error can be obtained from ERR_get_error(3).
The trusted certificate store is not searched for the signer's certificates. This is primarily due to the inadequacies of the current X509_STORE functionality.
The lack of single pass processing means that the signed content must all be held in memory if it is not detached.
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