Can I build an RSA public key from an OpenSSL configuration file?

Yes, you can build an RSA public key from an OpenSSL configuration file, if you know the components of the key, as shown in the test below:

C:\Users\fyicenter>type asn1_rsa_pub_key.cnf
asn1 = SEQUENCE:rsa_pub_key

[rsa_pub_key]
algorithm =SEQUENCE:rsa_alg
pubkey    =BITWRAP,SEQUENCE:rsa_pub_components

[rsa_alg]
algorithm =OID:rsaEncryption
parameter =NULL

[rsa_pub_components]
n         =INTEGER:0xBB6FE79432CC6EA2D8F970675A5A87BFBE1AFF0BE63E879F2AF\
FB93644D4D2C6D000430DEC66ABF47829E74B8C5108623A1C0EE8BE217B3AD8D36D5EB4FCA1D9
e         =INTEGER:0x010001


C:\Users\fyicenter>\local\openssl\openssl 
   asn1parse -genconf asn1_rsa_pub_key.cnf -noout -out rsa_pub_key.der

C:\Users\fyicenter>\local\openssl\openssl 
   pkey -pubin -in rsa_pub_key.der -inform DER -text -noout
Public-Key: (512 bit)
Modulus:
    00:bb:6f:e7:94:32:cc:6e:a2:d8:f9:70:67:5a:5a:
    87:bf:be:1a:ff:0b:e6:3e:87:9f:2a:ff:b9:36:44:
    d4:d2:c6:d0:00:43:0d:ec:66:ab:f4:78:29:e7:4b:
    8c:51:08:62:3a:1c:0e:e8:be:21:7b:3a:d8:d3:6d:
    5e:b4:fc:a1:d9
Exponent: 65537 (0x10001)

⇒ OpenSSL "ans1parse" - DSA Private Key in ASN.1 Structure

⇐ OpenSSL "ans1parse" - RSA Public Key in ASN.1 Structure

⇑ OpenSSL "ans1parse" Command

⇑⇑ OpenSSL Tutorials