fix 543: improve COSE_Key spec language and add COSE_Key examples #732

equalsJeffH · 2017-12-22T00:47:45Z

this builds on PR #731 to improve COSE_Key spec language and add COSE_Key examples

akshayku

Looks good to me.

akshayku · 2017-12-22T19:02:52Z

index.bs

+    -2:  e   ; e:   RSA public exponent e byte string 3 bytes in length
+                    e.g., in hex: 010001
+  }
+</pre>


One example:

{1: 3, 3: -257, -1: h'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', -2: h'010001'}

equalsJeffH · 2018-01-01T18:41:21Z

the build errors just now were both "disk quota exceeded":
[0Koci runtime error: exec failed: container_linux.go:265: starting container process caused "could not create session key: disk quota exceeded"
[I built the spec successfully locally using cloud bikeshed]

@jcjones -- help?

emlun

I'm not very familiar with these formats and encodings, so I'll abstain from reviewing.

agl · 2018-01-04T00:14:33Z

index.bs

+  {
+     1:  3,  ; kty: RSA key type
+     3:-37,  ; alg: RS256
+    -1:  n,  ; n:   RSA modulus n byte string 257 bytes in length


Should this value (and below) be a 256 byte value? (There's no zero padding to avoid negative numbers in COSE.)

thx hm. I dunno, you guys tell me. Here's how I generated the example RSA key pair, and openssl sez the modulus is 257 bytes:

bash-3.2$ openssl genrsa -des3 -out private.pem 2048 Generating RSA private key, 2048 bit long modulus .......................+++ ..........................................................................................................+++ e is 65537 (0x10001) Enter pass phrase for private.pem: Verifying - Enter pass phrase for private.pem: bash-3.2$ bash-3.2$ bash-3.2$ openssl rsa -in private.pem -outform PEM -pubout -out public.pem Enter pass phrase for private.pem: writing RSA key bash-3.2$ bash-3.2$ bash-3.2$ PUBKEY=`grep -v -- ----- public.pem | tr -d '\n'` bash-3.2$ bash-3.2$ bash-3.2$ echo $PUBKEY | base64 -D | openssl asn1parse -inform DER -i 0:d=0 hl=4 l= 290 cons: SEQUENCE 4:d=1 hl=2 l= 13 cons: SEQUENCE 6:d=2 hl=2 l= 9 prim: OBJECT :rsaEncryption 17:d=2 hl=2 l= 0 prim: NULL 19:d=1 hl=4 l= 271 prim: BIT STRING bash-3.2$ bash-3.2$ bash-3.2$ bash-3.2$ echo $PUBKEY | base64 -D | openssl asn1parse -inform DER -i -strparse 19 0:d=0 hl=4 l= 266 cons: SEQUENCE 4:d=1 hl=4 l= 257 prim: INTEGER :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 265:d=1 hl=2 l= 3 prim: INTEGER :010001

is there any zero-padding in the above modulus value? Ah, is the leading : padding (that COSE_Key encoding omits)? Or is the : an artifact of ASN.1 encoding? I checked the modulus length and it is 256 bytes without the :. But the length of the exponent is given as 3 bytes and it is shown with the : byte/char, so I'm not sure what is going on.

2048 bits / 8 = 256, so we would expect a 256-byte value.

ASN.1 DER requires that an INTEGER with the most-significant bit set be padded with a leading zero to avoid it appearing to be negative. Since the leading byte is 0xdb, that will have happened in this case. In fact, it'll happen for all RSA keys since the MSB is required to be 1 (otherwise it would be a 2047-bit key).

COSE, however, doesn't have this leading padding byte. That is made explicit here:

The octet sequence MUST utilize the minimum number of octets needed to represent the value.

Ok, thx, so is openssl just not displaying a leading zero padding byte? ( I think yes )

Correct, as it's not considered part of the value. If you base64-decode the public key and look for the modulus in a hex dump, it should be preceded by 0282010100.

agl · 2018-01-04T00:16:20Z

index.bs

+<pre class="example" highlight="json">
+  {
+     1:  3,  ; kty: RSA key type
+     3:-37,  ; alg: RS256


nit: PS256, rather than RS256, I believe.

yes, fixed, thx.

agl · 2018-01-04T00:20:11Z

index.bs

+<pre class="example" highlight="json">
+  {
+     1:   3,  ; kty: RSA key type
+     3:-257,  ; alg: RS256


Is algorithm -257 defined for COSE? I don't see it in the IANA registry. (In fact, I don't see PKCS #1 v1.5 at all.)

It's registered in WebAuthn, actually: https://w3c.github.io/webauthn/#sctn-cose-alg-reg

Once we reach CR, I plan to submit the registration request to IANA.

agl · 2018-01-04T00:23:28Z

(I see that I commented on the wrong subchange, but I think my point about s/257/256/ is still valid even if GitHub thinks it's outdated.)

equalsJeffH · 2018-01-10T22:09:37Z

@agl

ok, you are referring to #732 (comment), i believe.

I think I understand, tho I remain unsure whether the correct modulus length is 256 or 257 bytes, please see: #732 (comment), thx.

…an 257

equalsJeffH · 2018-01-10T22:48:13Z

Ready to merge?

agl · 2018-01-10T23:05:09Z

index.bs

@@ -2528,7 +2528,7 @@ to be used with the RSASSA-PSS w/ SHA-256 signature algorithm

 Below is an example of a COSE_Key-encoded 2048-bit RSA public key (see [[RFC8230]]
 [Section 4](https://tools.ietf.org/html/rfc8230#section-4))
-to be used with the RSASSA-PKCS1-v1_5 w/ SHA-256 signature algorithm (see [[#sctn-cose-alg-reg]]):
+to be used with the RS256 signature algorithm (RSASSA-PKCS1-v1_5 w/ SHA-256, see [[#sctn-cose-alg-reg]]):


(Maybe replace "w/" with "with". Your call.)

agl · 2018-01-10T23:07:36Z

index.bs

+  {
+     1:   3,  ; kty: RSA key type
+     3:-257,  ; alg: RS256
+    -1:  n,  ; n:   RSA modulus n byte string 257 bytes in length


I think one extra space is needed after the colon here and below. (And then the continued comment lines would need an extra space too.)

agl · 2018-01-10T23:08:25Z

index.bs

+  }
+</pre>
+
+Below is an example of a COSE_Key-encoded 2048-bit RSA public key (see [[RFC8230]]


(Could also just say that it's the same key because it's also RSA. Your call.)

…nd add COSE_Key examples (#732)

JeffH added 3 commits December 21, 2017 09:13

update COSE_Key format description parag

8b64e83

Merge branch 'master' into jeffh-fix-543-COSE_KEY-examples

c11f1b0

create example public keys section

1134d40

equalsJeffH added subtype:CTAP type:technical labels Dec 22, 2017

equalsJeffH added this to the CR milestone Dec 22, 2017

equalsJeffH self-assigned this Dec 22, 2017

equalsJeffH requested review from emlun, selfissued and akshayku December 22, 2017 00:47

akshayku approved these changes Dec 22, 2017

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alg -37 is actually PS256; align PS256 & RS256 text with ES256 text

66b2b7f

emlun reviewed Jan 3, 2018

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agl reviewed Jan 4, 2018

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JeffH added 2 commits January 10, 2018 14:35

Merge branch 'master' into jeffh-fix-543-COSE_KEY-examples

bcc6bd9

fix modulus length, as encoded in COSE_Key, to be 256 bytes rather th…

30de9f8

…an 257

agl approved these changes Jan 10, 2018

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selfissued approved these changes Jan 11, 2018

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JeffH added 4 commits January 11, 2018 15:27

Merge branch 'master' into jeffh-fix-543-COSE_KEY-examples

7908be8

incorp agl's comments, thx!

90ec096

adj example whitespace, thx agl!

d324d69

further whitespace twiddling...

0bac86e

equalsJeffH merged commit 58e824a into master Jan 12, 2018

WebAuthnBot pushed a commit that referenced this pull request Jan 12, 2018

Built by Travis-CI: 58e824a fix 543: improve COSE_Key spec language a…

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…nd add COSE_Key examples (#732)

emlun deleted the jeffh-fix-543-COSE_KEY-examples branch June 12, 2019 11:27

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fix 543: improve COSE_Key spec language and add COSE_Key examples #732

fix 543: improve COSE_Key spec language and add COSE_Key examples #732

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fix 543: improve COSE_Key spec language and add COSE_Key examples #732

fix 543: improve COSE_Key spec language and add COSE_Key examples #732

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