The smime command handles S/MIME mail. It can encrypt, decrypt, sign and verify S/MIME messages.
There are six operation options that set the type of operation to be performed. The meaning of the other options varies according to the operation type.
encrypt mail for the given recipient certificates. Input file is the message to be encrypted. The output file is the encrypted mail in MIME format.
Note that no revocation check is done for the recipient cert, so if that key has been compromised, others may be able to decrypt the text.
decrypt mail using the supplied certificate and private key. Expects an encrypted mail message in MIME format for the input file. The decrypted mail is written to the output file.
sign mail using the supplied certificate and private key. Input file is the message to be signed. The signed message in MIME format is written to the output file.
verify signed mail. Expects a signed mail message on input and outputs the signed data. Both clear text and opaque signing is supported.
takes an input message and writes out a PEM encoded PKCS#7 structure.
resign a message: take an existing message and one or more new signers.
the input message to be encrypted or signed or the MIME message to be decrypted or verified.
this specifies the input format for the PKCS#7 structure. The default is SMIME which reads an S/MIME format message. PEM and DER format change this to expect PEM and DER format PKCS#7 structures instead. This currently only affects the input format of the PKCS#7 structure, if no PKCS#7 structure is being input (for example with -encrypt or -sign) this option has no effect.
the message text that has been decrypted or verified or the output MIME format message that has been signed or verified.
this specifies the output format for the PKCS#7 structure. The default is SMIME which write an S/MIME format message. PEM and DER format change this to write PEM and DER format PKCS#7 structures instead. This currently only affects the output format of the PKCS#7 structure, if no PKCS#7 structure is being output (for example with -verify or -decrypt) this option has no effect.
the -stream and -indef options are equivalent and enable streaming I/O for encoding operations. This permits single pass processing of data without the need to hold the entire contents in memory, potentially supporting very large files. Streaming is automatically set for S/MIME signing with detached data if the output format is SMIME it is currently off by default for all other operations.
disable streaming I/O where it would produce and indefinite length constructed encoding. This option currently has no effect. In future streaming will be enabled by default on all relevant operations and this option will disable it.
This specifies a file containing the detached content, this is only useful with the -verify command. This is only usable if the PKCS#7 structure is using the detached signature form where the content is not included. This option will override any content if the input format is S/MIME and it uses the multipart/signed MIME content type.
this option adds plain text (text/plain) MIME headers to the supplied message if encrypting or signing. If decrypting or verifying it strips off text headers: if the decrypted or verified message is not of MIME type text/plain then an error occurs.
a file containing trusted CA certificates, only used with -verify.
a directory containing trusted CA certificates, only used with -verify. This directory must be a standard certificate directory: that is a hash of each subject name (using x509 -hash) should be linked to each certificate.
digest algorithm to use when signing or resigning. If not present then the default digest algorithm for the signing key will be used (usually SHA1).
the encryption algorithm to use. For example DES (56 bits) - -des, triple DES (168 bits) - -des3, EVP_get_cipherbyname() function) can also be used preceded by a dash, for example -aes_128_cbc. See enc for list of ciphers supported by your version of OpenSSL.
If not specified triple DES is used. Only used with -encrypt.
when verifying a message normally certificates (if any) included in the message are searched for the signing certificate. With this option only the certificates specified in the -certfile option are used. The supplied certificates can still be used as untrusted CAs however.
do not verify the signers certificate of a signed message.
do not do chain verification of signers certificates: that is don't use the certificates in the signed message as untrusted CAs.
don't try to verify the signatures on the message.
when signing a message the signer's certificate is normally included with this option it is excluded. This will reduce the size of the signed message but the verifier must have a copy of the signers certificate available locally (passed using the -certfile option for example).
normally when a message is signed a set of attributes are included which include the signing time and supported symmetric algorithms. With this option they are not included.
normally the input message is converted to ``canonical'' format which is effectively using CR and LF as end of line: as required by the S/MIME specification. When this option is present no translation occurs. This is useful when handling binary data which may not be in MIME format.
when signing a message use opaque signing: this form is more resistant to translation by mail relays but it cannot be read by mail agents that do not support S/MIME. Without this option cleartext signing with the MIME type multipart/signed is used.
allows additional certificates to be specified. When signing these will be included with the message. When verifying these will be searched for the signers certificates. The certificates should be in PEM format.
a signing certificate when signing or resigning a message, this option can be used multiple times if more than one signer is required. If a message is being verified then the signers certificates will be written to this file if the verification was successful.
the recipients certificate when decrypting a message. This certificate must match one of the recipients of the message or an error occurs.
the private key to use when signing or decrypting. This must match the corresponding certificate. If this option is not specified then the private key must be included in the certificate file specified with the -recip or -signer file. When signing this option can be used multiple times to specify successive keys.
one or more certificates of message recipients: used when encrypting a message.
the relevant mail headers. These are included outside the signed portion of a message so they may be included manually. If signing then many S/MIME mail clients check the signers certificate's email address matches that specified in the From: address.
Set various options of certificate chain verification. See verify manual page for details.
The MIME message must be sent without any blank lines between the headers and the output. Some mail programs will automatically add a blank line. Piping the mail directly to sendmail is one way to achieve the correct format.
The supplied message to be signed or encrypted must include the necessary MIME headers or many S/MIME clients wont display it properly (if at all). You can use the -text option to automatically add plain text headers.
A ``signed and encrypted'' message is one where a signed message is then encrypted. This can be produced by encrypting an already signed message: see the examples section.
This version of the program only allows one signer per message but it will verify multiple signers on received messages. Some S/MIME clients choke if a message contains multiple signers. It is possible to sign messages ``in parallel'' by signing an already signed message.
The options -encrypt and -decrypt reflect common usage in S/MIME clients. Strictly speaking these process PKCS#7 enveloped data: PKCS#7 encrypted data is used for other purposes.
The -resign option uses an existing message digest when adding a new signer. This means that attributes must be present in at least one existing signer using the same message digest or this operation will fail.
The -stream and -indef options enable experimental streaming I/O support. As a result the encoding is BER using indefinite length constructed encoding and no longer DER. Streaming is supported for the -encrypt operation and the -sign operation if the content is not detached.
Streaming is always used for the -sign operation with detached data but since the content is no longer part of the PKCS#7 structure the encoding remains DER.
an error occurred creating the PKCS#7 file or when reading the MIME message.
the message was verified correctly but an error occurred writing out the signers certificates.
Note: the encryption command does not include the -text option because the message being encrypted already has MIME headers.
The MIME parser isn't very clever: it seems to handle most messages that I've thrown at it but it may choke on others.
The code currently will only write out the signer's certificate to a file: if the signer has a separate encryption certificate this must be manually extracted. There should be some heuristic that determines the correct encryption certificate.
Ideally a database should be maintained of a certificates for each email address.
The code doesn't currently take note of the permitted symmetric encryption algorithms as supplied in the SMIMECapabilities signed attribute. This means the user has to manually include the correct encryption algorithm. It should store the list of permitted ciphers in a database and only use those.
No revocation checking is done on the signer's certificate.
The current code can only handle S/MIME v2 messages, the more complex S/MIME v3 structures may cause parsing errors.