Get .crt & .key files from .cer file

The following command is used to sign a string via openssl:

smime -sign -signer cert.crt -inkey key.key -engine gost -binary -noattr

Given a .cer file that was said to contain key within self how to I execute the same command?

I'd assume 2 options:

1. Rewrite the command to use internal key
2. Convert .cer file to .crt and extract .key from it to be used w/o rewriting existing command

Update: The file insides are as follows: (using openssl x509 -in cert.cer -noout -text, replaced sensitive data with X_X_X):

Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            X_X_X
        Signature Algorithm: GOST R 34.10-2012 with GOST R 34.11-2012 (256 bit)
        Issuer: X_X_X
        Validity
            X_X_X
            X_X_X
        Subject: X_X_X
        Subject Public Key Info:
            Public Key Algorithm: GOST R 34.10-2012 with 256 bit modulus
            Unable to load Public Key
842D0000:error:03000072:digital envelope routines:X509_PUBKEY_get0:decode error:crypto\x509\x_pubkey.c:458:
842D0000:error:03000072:digital envelope routines:X509_PUBKEY_get0:decode error:crypto\x509\x_pubkey.c:458:
        X509v3 extensions:
            X509v3 Key Usage: critical
                Digital Signature, Non Repudiation, Key Encipherment, Data Encipherment, Key Agreement
            X509v3 Subject Alternative Name:
                othername: title::<unsupported>
            X509v3 Extended Key Usage:
                TLS Web Server Authentication, TLS Web Client Authentication, 1.2.643.2.1.6.8.5, 1.2.643.3.61.502710.1.6.3.2, 1.2.643.3.251.1.1, 1.2.643.3.251.3
            X509v3 Certificate Policies:
                Policy: Class of Signing Tool KC1
            1.2.643.100.114:
                ...
            Signing Tool of Subject:
                X_X_X
            Signing Tool of Issuer:
                signTool    : X_X_X
                cATool      : X_X_X
                signToolCert: X_X_X
                cAToolCert  : X_X_X
            X509v3 CRL Distribution Points:
                Full Name:
                  X_X_X
                Full Name:
                  X_X_X
            Authority Information Access:
                CA Issuers - X_X_X
                CA Issuers - X_X_X
            X509v3 Subject Key Identifier:
                X_X_X
            X509v3 Authority Key Identifier:
                keyid:X_X_X
                DirName:X_X_X
                serial:X_X_X
    Signature Algorithm: GOST R 34.10-2012 with GOST R 34.11-2012 (256 bit)
    Signature Value:
        X_X_X

I guess there are several things to be noted.

The file suffix

First of all, it should be noted that there is nothing like "a .crt or .cer file" as suggested in the question. These are just file suffixes from which you cannot derive the actual file format.

For example, a .cer file created in Windows will usually be in DER (binary) format, while openssl defaults to PEM (base-64 text) format. The very same is true for for .crt which is just more commonly used suffix in Windows. Other suffixes are .der and .pem which suggest that their suffix and format match.

openssl can handle both formats. You can let openssl know the input format using the -inform der or -inform pem option.

The private key

I assume you want to sign an email message, because you are using the smime option (although you speak of signing a string, but that may just be the same in you context).

In order to sign a message, you need a private key. If the S/MIME certificate file you have been given obviously includes the private key (sometimes referred to as a private certificate), it should not be necessary to extract the key first. Just do not use the -inkey option.

Quoted from the openssl smime docs:

If this option is not specified then the private key must be included in the certificate file specified with the -recip or -signer file.

To create a cleartext signed message using a certificate in PEM format, use

openssl smime -sign -in message.txt -text -out mail.msg -signer cert.crt

The certificate file cannot be in DER format if it includes the private key, because the DER format can hold only 1 object, so certificate and private key would need to be in separate DER files. See also this answer about the difference between PEM and DER. (Thanks to dave_thompson_085 for correcting me.)