How to how extra an ASN.1 child structure using the OpenSSL "ans1parse" command?

You can use the OpenSSL "ans1parse -strparse offset" command to extract an ASN.1 child structure from a nested ASN.1 structure. as shown in the test below:

C:\Users\fyicenter>\local\openssl\openssl.exe

OpenSSL> asn1parse -inform DER -in asn1_question.der -i
    0:d=0  hl=2 l= 105 cons: SEQUENCE
    2:d=1  hl=2 l=   1 prim:  INTEGER           :05
    5:d=1  hl=2 l=  53 prim:  IA5STRING         :In asymmetric encryption, 
                                                 which key can be given out?
   60:d=1  hl=2 l=  45 cons:  SEQUENCE
   62:d=2  hl=2 l=  14 prim:   IA5STRING         :A) Private key
   78:d=2  hl=2 l=  13 prim:   IA5STRING         :B) Public key
   93:d=2  hl=2 l=  12 prim:   IA5STRING         :C) Both keys

OpenSSL> asn1parse -inform DER -in asn1_question.der -i -strparse 60
    0:d=0  hl=2 l=  45 cons: SEQUENCE
    2:d=1  hl=2 l=  14 prim:  IA5STRING         :A) Private key
   18:d=1  hl=2 l=  13 prim:  IA5STRING         :B) Public key
   33:d=1  hl=2 l=  12 prim:  IA5STRING         :C) Both keys

OpenSSL> asn1parse -inform DER -in asn1_question.der -i -strparse 60 
   -noout -out asn1_question_option.der

OpenSSL> asn1parse -inform DER -in asn1_question_option.der
    0:d=0  hl=2 l=  45 cons: SEQUENCE
    2:d=1  hl=2 l=  14 prim: IA5STRING         :A) Private key
   18:d=1  hl=2 l=  13 prim: IA5STRING         :B) Public key
   33:d=1  hl=2 l=  12 prim: IA5STRING         :C) Both keys

OpenSSL> asn1parse -inform DER -in asn1_question_option.der -strparse 33
Error in encoding
12608:error:0D07209B:asn1 encoding routines:ASN1_get_object:too long:
.\crypto\asn1\asn1_lib.c:147:   

Note about the test:

• "asn1parse -inform DER -in asn1_question.der -i" command parses a nested SEQUENCE ASN.1 structure.
• "asn1parse -inform DER -in asn1_question.der -i -strparse 60" command extracts the child SEQUENCE ASN.1 structure at offset of 0x60.
• "asn1parse -inform DER -in asn1_question.der -i -strparse 60 -noout -out asn1_question_option.der" command extracts the child SEQUENCE ASN.1 structure at offset of 0x60 and saves the child structure to the output DER file.
• "asn1parse -inform DER -in asn1_question_option.der" command parses the child SEQUENCE structure from the DER file.
• "asn1parse -inform DER -in asn1_question_option.der -strparse 33" command tries to extract a primitive field. But this is not supported.

⇒ OpenSSL "ans1parse" - ASN.1 INTEGER Field Type

⇐ OpenSSL "ans1parse -genconf" - Nested SEQUENCE ASN.1 Structure

⇑ OpenSSL "ans1parse" Command

⇑⇑ OpenSSL Tutorials