OpenPGP: Support for Curve25519, Ed25519, X448 and Ed448

Hey!

As far as I can tell, BC is missing support for some algorithms. Also, there are ambiguities regarding the used curve OIDs. Here are my observations, note that I'm not an expert, so my observations might be wrong:

### BCs Curve Constants

I'm comparing the curve OIDs to those from [crypto-refresh-05](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-05.html#section-9.2):

| RFC 4880 Algo Name | BC OID Constant                         | Value                  |
|--------------------|-----------------------------------------|------------------------|
| Curve25519 (ECDH)  | `CryptlibObjectIdentifiers.curvey25519` | 1.3.6.1.4.1.3029.1.5.1 |
| not used                 | `EdECObjectIdentifiers.id_X25519`       | 1.3.101.110            |
| X448 (ECDH)        | `EdECObjectIdentifiers.id_X448`         | 1.3.101.111            |
| not used                  | `EdECObjectIdentifiers.id_Ed25519`      | 1.3.101.112            |
| Ed448 (EdDSA)      | `EdECObjectIdentifiers.id_Ed448`        | 1.3.101.113            |
| Ed25519 (EdDSA)    | `GNUObjectIdentifiers.Ed25519`          | 1.3.6.1.4.1.11591.15.1 |


### JcaPGPKeyConverter
| *Algorithm*       | getPrivateKey()         | getPublicKey()          | getPrivateBCPGKey() | getPublicBCPGKey() |
|-------------------|-------------------------|-------------------------|---------------------|--------------------|
| Curve25519 (ECDH) | :white_check_mark: [^1] | :white_check_mark: [^3] | Probably? [^5]      | :heavy_check_mark: |
| X448 (ECDH)       | :x:                     | :x:                     | Probably not? [^5]  | :x:                |
| Ed25519 (EdDSA)   | :heavy_check_mark: [^2] | :heavy_check_mark: [^4] | Maybe? [^6]         | :heavy_check_mark: |
| Ed448 (EdDSA)     | :x:                     | :x:                     | Probably not? [^6]  | :x:                |

[^1]: Comments mention XDH, refer to `EdECObjectIdentifiers.id_X25519`, should use `CryptlibObjectIdentifiers.curvey25519`? ([src](https://github.com/bcgit/bc-java/blob/1349bb03cff0b0023ef3247690410441f33b05b0/pg/src/main/java/org/bouncycastle/openpgp/operator/jcajce/JcaPGPKeyConverter.java#L194-L197))
[^2]: Refers to `EdECObjectIdentifiers.id_Ed25519`, should use `GNUObjectIdentifiers.Ed25519`? ([src](https://github.com/bcgit/bc-java/blob/1349bb03cff0b0023ef3247690410441f33b05b0/pg/src/main/java/org/bouncycastle/openpgp/operator/jcajce/JcaPGPKeyConverter.java#L212-L214))
[^3]: Refers to `EdECObjectIdentifiers.id_X25519` in method call, should use `CryptlibObjectIdentifiers.curvey25519`? ([src](https://github.com/bcgit/bc-java/blob/1349bb03cff0b0023ef3247690410441f33b05b0/pg/src/main/java/org/bouncycastle/openpgp/operator/jcajce/JcaPGPKeyConverter.java#L282-L284))
[^4]: Refers to `EdECObjectIdentifiers.id_Ed25519`, should use `CryptlibObjectIdentifiers.curvey25519`? ([src](https://github.com/bcgit/bc-java/blob/1349bb03cff0b0023ef3247690410441f33b05b0/pg/src/main/java/org/bouncycastle/openpgp/operator/jcajce/JcaPGPKeyConverter.java#L307-L309))
[^5]: Source is not clear, does not compare OIDs ([src](https://github.com/bcgit/bc-java/blob/1349bb03cff0b0023ef3247690410441f33b05b0/pg/src/main/java/org/bouncycastle/openpgp/operator/jcajce/JcaPGPKeyConverter.java#L370-L392))
[^6]: Source is not clear, does not compare OIDs ([src](https://github.com/bcgit/bc-java/blob/1349bb03cff0b0023ef3247690410441f33b05b0/pg/src/main/java/org/bouncycastle/openpgp/operator/jcajce/JcaPGPKeyConverter.java#L400-L413))


### BcPGPKeyConverter
| *Algorithm*       | getPrivateKey()         | getPublicKey()           | getPrivateBCPGKey()      | getPublicBCPGKey()       |
|-------------------|-------------------------|--------------------------|--------------------------|--------------------------|
| Curve25519 (ECDH) | :white_check_mark: [^7] | :white_check_mark: [^9]  | :white_check_mark: [^11] | :white_check_mark: [^12] |
| X448 (ECDH)       | :x:                     | :x:                      | :x:                      | :heavy_check_mark:       |
| Ed25519 (EdDSA)   | :white_check_mark: [^8] | :heavy_check_mark: [^10] | :heavy_check_mark:       | :heavy_check_mark:       |
| Ed448 (EdDSA)     | :heavy_check_mark:      | :heavy_check_mark:       | :heavy_check_mark:       | :heavy_check_mark:       |

[^7]: Comments mention X25519, refers to `EdECObjectIdentifiers.id_X25519`, should use `CryptlibObjectIdentifiers.curvey25519`? ([src](https://github.com/bcgit/bc-java/blob/1349bb03cff0b0023ef3247690410441f33b05b0/pg/src/main/java/org/bouncycastle/openpgp/operator/bc/BcPGPKeyConverter.java#L125-L127))
[^8]: Refers to `EdECObjectIdentifiers.id_Ed25519`, should use `GNUObjectIdentifiers.Ed25519`? ([src](https://github.com/bcgit/bc-java/blob/1349bb03cff0b0023ef3247690410441f33b05b0/pg/src/main/java/org/bouncycastle/openpgp/operator/bc/BcPGPKeyConverter.java#L150-L152))
[^9]: Refers to `EdECObjectIdentifiers.id_X25519`, should use `CryptlibObjectIdentifiers.curvey25519`? ([src](https://github.com/bcgit/bc-java/blob/1349bb03cff0b0023ef3247690410441f33b05b0/pg/src/main/java/org/bouncycastle/openpgp/operator/bc/BcPGPKeyConverter.java#L217-L219))
[^10]: If statement is hard to read ([src](https://github.com/bcgit/bc-java/blob/1349bb03cff0b0023ef3247690410441f33b05b0/pg/src/main/java/org/bouncycastle/openpgp/operator/bc/BcPGPKeyConverter.java#L241))
[^11]: Comment and code mention "X25519", not clear if it uses Ed25519
[^12]: `pubkey instanceof X25519PublicKeyParameters`, is this correct for `Curve25519`?





Algorithm	getPrivateKey()	getPublicKey()	getPrivateBCPGKey()	getPublicBCPGKey()
Curve25519 (ECDH)	✅ ¹	✅ ²	Probably? ³	✔️
X448 (ECDH)	❌	❌	Probably not? ³	❌
Ed25519 (EdDSA)	✔️ ⁴	✔️ ⁵	Maybe? ⁶	✔️
Ed448 (EdDSA)	❌	❌	Probably not? ⁶	❌

Algorithm	getPrivateKey()	getPublicKey()	getPrivateBCPGKey()	getPublicBCPGKey()
Curve25519 (ECDH)	✅ ⁷	✅ ⁸	✅ ⁹	✅ ¹⁰
X448 (ECDH)	❌	❌	❌	✔️
Ed25519 (EdDSA)	✅ ¹¹	✔️ ¹²	✔️	✔️
Ed448 (EdDSA)	✔️	✔️	✔️	✔️

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OpenPGP: Support for Curve25519, Ed25519, X448 and Ed448 #1142

BCs Curve Constants

JcaPGPKeyConverter

BcPGPKeyConverter

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RFC 4880 Algo Name	BC OID Constant	Value
Curve25519 (ECDH)	`CryptlibObjectIdentifiers.curvey25519`	1.3.6.1.4.1.3029.1.5.1
not used	`EdECObjectIdentifiers.id_X25519`	1.3.101.110
X448 (ECDH)	`EdECObjectIdentifiers.id_X448`	1.3.101.111
not used	`EdECObjectIdentifiers.id_Ed25519`	1.3.101.112
Ed448 (EdDSA)	`EdECObjectIdentifiers.id_Ed448`	1.3.101.113
Ed25519 (EdDSA)	`GNUObjectIdentifiers.Ed25519`	1.3.6.1.4.1.11591.15.1

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OpenPGP: Support for Curve25519, Ed25519, X448 and Ed448 #1142

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BCs Curve Constants

JcaPGPKeyConverter

BcPGPKeyConverter

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