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ISBN : 978-2-7460-9712-4
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CentOS 2.1AS







smime − S/MIME utility


openssl smime [−encrypt] [−decrypt] [−sign] [−verify] [−pk7out] [−des] [−des3] [−rc2-40] [−rc2-64] [−rc2-128] [−in file] [−certfile file] [−signer file] [−recip file] [−in file] [−inform SMIME⎪PEM⎪DER] [−passin arg] [−inkey file] [−out file] [−outform SMIME⎪PEM⎪DER] [−content file] [−to addr] [−from ad] [−subject s] [−text] [−rand file(s)] [cert.pem]...


The smime command handles S/MIME mail. It can encrypt, decrypt, sign and verify S/MIME messages.


There are five 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.


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.

−in filename

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.

−out filename

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.

−content filename

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.

−CAfile file

a file containing trusted CA certificates, only used with −verify.

−CApath dir

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.

−des −des3 −rc2-40 −rc2-64 −rc2-128

the encryption algorithm to use. DES (56 bits), triple DES (168 bits) or 40, 64 or 128 bit RC2 respectively if not specified 40 bit RC2 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.

−certfile file

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.

−signer file

the signers certificate when signing a message. If a message is being verified then the signers certificates will be written to this file if the verification was successful.

−recip file

the recipients certificate when decrypting a message. This certificate must match one of the recipients of the message or an error occurs.

−inkey file

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.

−passin arg

the private key password source. For more information about the format of arg see the PASS PHRASE ARGUMENTS section in openssl(1).

−rand file(s)

a file or files containing random data used to seed the random number generator, or an EGD socket (see RAND_egd(3)). Multiple files can be specified separated by a OS −dependent character. The separator is ; for MS −Windows, , for OpenVMS, and : for all others.


one or more certificates of message recipients: used when encrypting a message.

−to, −from, −subject

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.


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 operation was completely successfully.


an error occurred parsing the command options.


one of the input files could not be read.


an error occurred creating the PKCS #7 file or when reading the MIME message.


an error occurred decrypting or verifying the message.


the message was verified correctly but an error occurred writing out the signers certificates.


Create a cleartext signed message:

 openssl smime -sign -in message.txt -text -out mail.msg \
        -signer mycert.pem

Create and opaque signed message

 openssl smime -sign -in message.txt -text -out mail.msg -nodetach \
        -signer mycert.pem

Create a signed message, include some additional certificates and read the private key from another file:

 openssl smime -sign -in in.txt -text -out mail.msg \
        -signer mycert.pem -inkey mykey.pem -certfile mycerts.pem

Send a signed message under Unix directly to sendmail, including headers:

 openssl smime -sign -in in.txt -text -signer mycert.pem \
        -from steve@openssl.org -to someone@somewhere \
        -subject "Signed message" ⎪ sendmail someone@somewhere

Verify a message and extract the signer’s certificate if successful:

 openssl smime -verify -in mail.msg -signer user.pem -out signedtext.txt

Send encrypted mail using triple DES:

 openssl smime -encrypt -in in.txt -from steve@openssl.org \
        -to someone@somewhere -subject "Encrypted message" \
        -des3 user.pem -out mail.msg

Sign and encrypt mail:

 openssl smime -sign -in ml.txt -signer my.pem -text \
        ⎪ openssl smime -encrypt -out mail.msg \
        -from steve@openssl.org -to someone@somewhere \
        -subject "Signed and Encrypted message" -des3 user.pem

Note: the encryption command does not include the −text option because the message being encrypted already has MIME headers.

Decrypt mail:

 openssl smime -decrypt -in mail.msg -recip mycert.pem -inkey key.pem

The output from Netscape form signing is a PKCS#7 structure with the detached signature format. You can use this program to verify the signature by line wrapping the base64 encoded structure and surrounding it with:

 -----BEGIN PKCS7----
 -----END PKCS7----

and using the command,

 openssl smime -verify -inform PEM -in signature.pem -content content.txt

alternatively you can base64 decode the signature and use

 openssl smime -verify -inform DER -in signature.der -content content.txt


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.