[OpenID-Specs-eKYC-IDA] Issue #1289: serial_number of Japanese Personal Number cannot be revealed (openid/ekyc-ida)

[Kosuke Koiwai]

New issue 1289: serial_number of Japanese Personal Number cannot be revealed
https://bitbucket.org/openid/ekyc-ida/issues/1289/serial_number-of-japanese-personal-number

Kosuke Koiwai:

Japanese National Personal Number system restricts the use of the serial number of the certificate, and the number can only be handled by companies certified by the government. Thus, this field should be OPTIONAL or allow the use of ppid-ish number that IdP assigns to the person/certificate. 

##### [5.1.1.5. ](https://openid.net/specs/openid-connect-4-identity-assurance-1_0-ID3.html#section-5.1.1.5)[Evidence Type electronic\_signature](https://openid.net/specs/openid-connect-4-identity-assurance-1_0-ID3.html#name-evidence-type-electronic_si)

```none
`serial_number`: REQUIRED. String containing the serial number of the certificate used to sign.
```

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